The mammalian Ras proteins are a family of guanosine triphosphate (GTP) binding and hydrolyzing proteins that regulate cell growth and differentiation. Their overproduction or mutation can lead to uncontrolled cell growth, and has been implicated as a cause or aggravating factor in a variety of diseases including cancer, restenosis, psoriasis, endometriosis, atherosclerosis, viral or yeast infection, and corneal neovascularization.
Ras proteins share characteristic C-terminal sequences termed the CAAX motif, wherein C is Cys, A is an amino acid, usually an aliphatic amino acid, and X is an aliphatic amino acid or other type of amino acid. The biological activity of the proteins is dependent upon the post-translational modification of these sequences by isoprenoid lipids. For proteins having a C-terminal CAAX sequence, this modification, which is called prenylation, occurs in at least three steps: the addition of either a 15 carbon (farnesyl) or 20 carbon (geranylgeranyl) isoprenoid to the Cys residue, the proteolytic cleavage of the last three amino acids from the C-terminus, and the methylation of the new C-terminal carboxylate. Zhang and Casey, Ann. Rev. Biochem. 1996, 65, 241–269. The prenylation of some proteins may include a fourth step; the palmitoylation of one or two Cys residues N-terminal to the farnesylated Cys.
Ras-like proteins terminating with XXCC or XCXC motifs can also be prenylated and are modified by geranylgeranylation on the Cys residues. These proteins do not require an endoproteolytic processing step. While some mammalian cell proteins terminating in XCXC are carboxymethylated, it is not clear whether carboxymethylation follows prenylation of proteins terminating with XXCC motifs. Clarke, Ann. Rev. Biochem., 1992, 61, 355–386. For all Ras-like proteins, however, addition of the isoprenoid is the first step of prenylation, and is required for the subsequent steps. Cox and Der, Critical Rev. Oncogenesis, 1992, 3, 365–400; and Ashby et al., Curr. Opinion Lipidology, 1998, 9, 99–102.
Three enzymes have been found to catalyze protein prenylation: farnesyl-protein transferase (FPTase), geranylgeranyl-protein transferase type I (GGPTase-I), and geranylgeranyl-protein transferase type-II (GGPTase-II, also called Rab GGPTase). These enzymes are present in both yeast and mammalian cells. Schafer and Rine, Annu. Rev. Genet., 1992, 30, 209–237. U.S. Pat. No. 5,578,477 discloses a method of purifying FPTase using recombinant technology and yeast host cells. Such techniques are useful in the elucidation of the enzyme structures.
FPTase and GGPTase-I are α/β heterodimeric enzymes that share a common α subunit; the β subunits are distinct but share approximately 30% amino acid identity. Brown and Goldstein, Nature, 1993, 366, 14–15; Zhang et al., J. Biol. Chem., 1994, 269, 3175–3180. GGPTase II has different α and β subunits, and complexes with a third component (REP, Rab Escort Protein) that presents the protein substrate to the α/β catalytic subunits. GGPTase proteins, and the nucleic acid sequence encoding them, are disclosed by U.S. Pat. No. 5,789,558 and WO 95/20651. U.S. Pat. No. 5,141,851 discloses the structure of a FPTase protein.
Each of these enzymes selectively uses farnesyl diphosphate or geranylgeranyl diphosphate as the isoprenoid donor, and selectively recognizes the protein substrate. FPTase farnesylates CAAX-containing proteins that end with Ser, Met, Cys, Gln or Ala. GGPTase-I geranylgeranylates CAAX-containing proteins that end with Leu or Phe. For FPTase and GGPTase-I, CAAX tetrapeptides comprise the minimum region required for interaction of the protein substrate with the enzyme. GGPTase-II modifies XXCC and XCXC proteins, but its interaction with protein substrates is more complex, requiring protein sequences in addition to the C-terminal amino acids for recognition. Enzymological characterization of FPTase, GGPTase-I and GGPTase-II has demonstrated that it is possible to selectively inhibit only one of these enzymes. Moores et al, J. Biol. Chem., 1991, 266, 17438.
GGPTase-I transfers a geranylgeranyl group from the prenyl donor geranylgeranyl diphosphate to the cysteine residue of substrate CAAX protein. Clarke, Annu. Rev. Biochem., 1992, 61, 355–386; Newman and Magee, Biochim. Biophys. Acta, 1993, 1155, 79–96. Known targets of GGPTase-I include the gamma subunits of brain heterotrimeric G proteins and Ras-related small GTP-binding proteins such as RhoA, RhoB, RhoC, CDC42Hs, Rac1, Rac2, Rap1A and Rap1B. The proteins RhoA, RhoB, RhoC, Rac1, Rac2 and CDC42Hs have roles in the regulation of cell shape. Ridley and Hall, Cell, 1992, 70, 389–399; Ridley et al., Cell, 1992, 70, 401–410; Bokoch and Der, FASEB J., 1993, 7, 750–759. Rac and Rap proteins play roles in neutrophil activation.
It has been found that the ability of Ras proteins to affect cell shape is dependant upon Rho and Rac protein function. See, e.g., Mackey and Hall, J. Biol. Chem., 1998, 273, 20688–20695. It thus follows that because Rho and Rac proteins require geranylgeranylation for function, an inhibitor of GGPTase-I would block the functions of these proteins, and may be useful as, for example, an anticancer agent. This notion is supported by recently reported research.
For example, GGPTase-I inhibitors can arrest human tumor cells that lack p53 in G0/G1, and induce the accumulation of p21WAP. This suggests that these inhibitors could be used to restore growth arrest in cells lacking functional p53. Vogt et al., J. Biol. Chem., 1997, 272, 27224–27229. Noteworthy in this regard are reports indicating that K-Ras, the form of Ras gene most associated with human cancers, can be modified by GGPTase-I in cells where FPTase is inhibited. Whyte et al., J. Biol. Chem., 1997, 272, 14459–14464. Since geranylgeranylated Ras has been reported to be as efficient as the farnesylated form in cell transformation studies, K-Ras cancers could be treated with GGPTase-I inhibitors. Lemer et al., J. Biol. Chem., 1995, 270, 26770–26773.
In addition to cancer, there are other pathological conditions for which GGPTase inhibitors may be used as intervention agents. These include, for example, the intimal hyperplasia associated with restenosis and atherosclerosis. Pulmonary artery smooth muscle cells seem particularly sensitive to inhibition of GGPTase-I, and treatment of such cells with a GGPTase inhibitor resulted in a superinduction of their inducible nitric-oxide synthase (NOS-2) by interleukin-1β. Finder et al., J. Biol. Chem., 1997, 272, 13484–13488.
GGPTase inhibitors may also be used as anti-fungal agents. In S. cerevisiae and Candida albicans, and apparently most other fungi, cell wall biosynthesis is controlled by a Rho-type protein that is modified by the fungal GGPTase-I. Qadota et al., Science, 1996, 272, 279–281. Selective inhibition of the fungal enzyme would diminish cell wall integrity, and thus be lethal to fungal cells.
Numerous other prenylation inhibitors have been studied. Some examples of these are disclosed by U.S. Pat. Nos. 5,420,245; 5,574,025; 5,523,430; 5,602,098; 5,631,401; 5,705,686; 5,238,922; 5,470,832; and 6,191,147; and by European Application Nos. 856,315 and 537,008. The effectiveness and specificity of these inhibitors vary widely, as do their chemical structures, and many of them are difficult to synthesize and purify.
Therefore, there is a need for new, more effective prenyl-protein transferase inhibitors.